Semantic memory in Alzheimer's disease: an analysis of category fluency.

In order to study the strategies of access to semantic knowledge and the status of semantic representation, we analyzed the performance in a semantic fluency task (Animal naming) of 70 patients with probable Alzheimer's disease (AD), including 40 mild AD patients and 30 moderate-severe AD patients, and 35 matched elderly normal control subjects. The total amount of words produced, their lexical frequency, the number and type of errors, as well as the presence and the type of clusters of related animal names were recorded. AD patients produced less animal exemplars than controls. In addition to a lower total number of clusters, the AD patients showed a strong limitation in clusters' selection, producing a limited number of different clusters. While normal subjects produced several different clusters, only farm animals were clustered by AD patients. When the total amount of words was considered, the mean lexical frequency was higher in the AD group. On the other hand, the words produced within clusters were comparable to terms of lexical frequency and number. These data confirm the impairment of AD patients in category fluency tasks, and suggest a degradation of semantic knowledge as the most likely underlying mechanism.